1. Field of the Invention
The present invention relates to a method of charging an integrated nickel/metal hydride or nickel/cadmium battery when the end of charging is determined by its temperature variation.
The invention finds one particularly advantageous application in the field of radiocommunication terminals, as shown in FIG. 1.
2. Description of the Prior Art
FIG. 1 shows a conventional radiocommunication terminal 1 essentially comprising a receive antenna 2, a screen 3, a navigation key 4 and a keypad 5. The terminal 1 further comprises an integral battery 6 and a connector 7 for connecting the battery 6 to a battery charger 8. The battery charger 8 is designed to be connected to a mains socket outlet 9. The terminal 1 finally comprises a printed circuit IMP that accommodates all of the circuits of the radiocommunication terminal. A device 10 for measuring the temperature of the battery 6 is disposed inside the battery.
The battery 6 is a nickel/metal hydride or nickel/cadmium battery.
The end of charging of nickel/metal hydride or nickel/cadmium batteries is detected by measuring the thermal slope representative of the evolution of the temperature of the battery as a function of time, which is typically of the order of a few degrees Celsius per minute. In fact, the end of charging of this type of battery is related to the temperature of the cells constituting them. Accordingly, when the battery reaches the end of charging, the temperature of the cells constituting the battery increases and charging is stopped. For implementing all the operations for detecting the end of charging, the radiocommunication terminal 1 includes a charging device, which is shown in FIG. 2. This is known in the art.
FIG. 2 shows a charging device 11 of the radiocommunication terminal 1, as used in the prior art. The charging device 11 includes the charger 8 which has two input terminals connected to the mains 12 via the plug 9. Two output terminals of the charger 8 are connected to the battery 6 via a charging switch 13. The charging device further includes a microprocessor 14 connected to the device 10 for measuring the battery temperature in order to command closing or opening of the charging switch 13 to start or stop charging the battery. The device 10 for measuring the battery temperature is advantageously disposed inside the battery.
The charger 8 has,the two-fold function of rectification and filtering in order to produce the appropriate current and voltage for charging the battery 6 from the mains power supply 12.
The temperature measuring device 10 measures the battery temperature. Accordingly, from information on the battery temperature supplied by the measuring device 10, the microprocessor 14 calculates the thermal slope of the battery, which is representative of the evolution of the temperature of the battery as a function of time. If the temperature of the cells constituting the battery increases, the microprocessor commands closing or opening of the charging switch to start or stop charging the battery.
Advances in mobile telephone technology are reflected in an increasing component integration density, and there is a trend towards a significant reduction in the size of handsets.
With very small handsets, the battery is very close to the components and consequently the thermal sensor intended to measure the evolution of the temperature associated with charging the battery is influenced by the power dissipation of the components around the battery. Their power dissipation increases the ambient temperature, i.e. the external temperature of the battery, which has repercussions on measurement of the battery temperature: the battery temperature increases without this always reflecting the end of charging of the battery. Accordingly, the influence of the battery""s environment can cause an erroneous end of charge condition to be detected, and consequently lead to defective charging of the battery.
The patent document U.S. Pat. No. 5,627,451 describes a method of circumventing the influence of ambient temperature and therefore of preventing erroneous detection of the end of battery charging. To manage the problem of the environment interfering with measuring the battery temperature, the method according to the aforementioned patent also entails measuring the battery voltage. The above method of controlling the charging of a battery consists of measuring, during charging, the variation xcex94V/xcex94t of the battery voltage as a function of time and the variation xcex94T/xcex94t of the battery temperature as a function of time. If a significant slope in the evolution of the battery temperature as a function of time is detected during charging but no change in the value xcex94V/xcex94t is detected, the charging of the battery of continued until the variation xcex94V/xcex94t of the voltage as a function of time and the variation xcex94T/xcex94t of the temperature of the battery as a function of time are at least greater than a predetermined variation of the battery voltage per unit time and a predetermined variation of the battery temperature per unit time, respectively.
The above method therefore entails using a first device to measure the battery voltage and a second device to measure the battery temperature. This solution also necessitates very accurate measurements in the case of nickel/metal hydride batteries. In fact, for nickel/metal hydride batteries the variations in the voltage as a function of time are very small and it is then difficult to detect a voltage slope. This two-fold necessity makes the method according to the patent previously cited costly.
Other methods use a first temperature sensor to detect variations in the ambient temperature and a second temperature sensor to detect variations in temperature associated only with charging the battery. The battery charging temperature is then corrected to take into account modifications due to the environment of the battery.
This type of method still has the disadvantage of requiring two measurement sensors, one of which is outside the battery.
Accordingly, the technical problem to be solved by the present invention is to alleviate the drawbacks of the prior art by providing a simple, low-cost method of thermally detecting the end of charging of a nickel/metal hydride or nickel/cadmium battery that reduces the influence of the environment of the battery and is reflected in control of the evolution of the temperature of the battery as a function of time during charging.
To this end, the solution to the stated technical problem in accordance with the present invention consists of using a single temperature sensor inside the battery to measure the variations in the temperature of the battery as a function of time during charging the battery. The function of the thermal sensor is to detect a first slope of value (X+Y) degrees Celsius per minute due to the power dissipation of the components around the battery and to the increase in the temperature of the battery cells and then to detect a second slope of value Y degrees Celsius in the evolution of the temperature of the battery as a function of time due only to an increase in the temperature of the battery cells signifying the end of charging.
Accordingly, the power dissipation of the components is taken into account by detecting a slope of value equal to X degrees Celsius per minute and the heating of the battery signifying the end of charging is taken into account by detecting a slope of value equal to Y degrees Celsius per minute. The slope values X and Y can be varied and are dependent on the type of radiocommunication terminal used and on the type of battery used. Nevertheless, they are typically within a range of values from a few tenths of a degree Celsius per minute to a few degrees Celsius per minute. The value X can be equal to 0.5xc2x0 C./minute and the value Y can be equal to 1xc2x0 C./minute, for example.
The invention therefore provides a method of charging a battery in which the end of charging is determined by detecting variations in the battery temperature, the method including the following steps:
a) entering a charging mode,
b) periodically measuring the battery temperature,
c) measuring the variation in the battery temperature per unit time,
d) comparing the battery temperature variation per unit time:
d1) to a first threshold corresponding to variation of the ambient temperature and the battery temperature, and in this case:
terminating the charging mode, and
d2) to variation of only the ambient temperature, and in this case:
comparing the battery temperature variation per unit time to a second threshold corresponding to variation of only the battery temperature, and
terminating the charging mode.
The invention also provides a radiocommunication terminal including a printed circuit and a charging device, which charging device includes a battery charger connected to a mains electrical power supply via a connector, to a battery via a charging switch, and to a microprocessor adapted to command the charging switch and connected to a temperature measuring device, in which terminal the microprocessor is adapted to implement the above method.
Other features and advantages of the invention will become more clearly apparent on reading the following description of one particular embodiment with reference to the accompanying drawings, which description is given by way of non-limiting example only.